This invention relates to a mobile communication system of a cellular type and, in particular, to a mobile communication system having cells hierarchized in accordance with a width of each service area.
In a large-capacity mobile communication system such as an automobile telephone system, effective utilization of frequencies is achieved by covering service areas by a plurality of base stations and by repeatedly using the same speech channel in the base stations where interference does not occur. Such a mobile communication system is called a cellular type mobile communication system.
In recent years, introduction of the cellular type mobile communication system having narrow service areas has been studied in order to increase the number of subscribers which the system can accommodate. Such a narrow service area is called a microcell. Micro-cellulation results in increasing the number of the subscribers but results in increasing expenses of installing the base stations per unit area. As a result, to construct all-of service areas by microcells is not realistic.
As a realistic solution, a hierarchical construction method is studied. In the hierarchical construction method, an area accommodating a lot of subscribers is micro-cellulared into a plurality of microcells and the microcells are covered by a wide service area. Such a wide service area is called a macrocell.
Various channel assignment methods of assigning a speech channel used in each base station are already known. One of the channel assignment methods is called a dynamic channel assignment method which selects, as a selected speech channel, one of the speech channels where interference does not occur in each communication and uses the selected speech channel. The dynamic channel assignment method is disadvantageous in that a control method and structure of apparatus are complicated but is advantageous in that it is possible to accommodate a lot of subscribers. This is because any speech channel is freely used insofar as the interference does not occur. Accordingly, the dynamic channel assignment method has been adopted in the automobile telephone system. In a hierarchical cellular type mobile communication system, use of the dynamic channel assignment method results in increasing the number of the subscribers which the system can accommodate and this method is effective from the viewpoint of effective utilization of frequencies.
The dynamic channel assignment method for use in the hierarchical cellular type mobile communication system is disclosed in Japanese Unexamined Patent Publication of Tokkai No. Hei 8-140,135 or JP-A 8-140,135 which is proposed by the present inventor and which has a title of "MOBILE COMMUNICATION SYSTEM".
Description will be made as regards operation of each base station in the mobile communication system according to JP-A 8-140,135. It is assumed as follows. The speech channels are N channels where N represents an integer which is not less than two. Selection order of speech channels is preliminarily set. In addition, in each cell, a usable speech channel is searched in order of high priority of the speech channels and one of the speech channels is selected as a selected speech channel. In order to avoid interference between hierarchies, the speech channel having a high utilization frequency in the same hierarchy is used in the opposite hierarchy with as low a frequency as possible. For this purpose, the base stations are classified into a plurality of groups of respective hierarchies. In addition, as mentioned above, the channel selection order assigned to each speech channel of each group is determined so as to use the speech channel having the high utilization frequency in the same group with as low a frequency as possible in the opposite group. It is possible. to determine a different channel selection order in each hierarchy. This is because the channel selection order can be preliminarily set prior to installation of the base stations.
To set the channel selection order in the above-mentioned manner results in realizing following characteristics in each hierarchy. A first characteristic is to decrease interference from the opposite hierarchy with the speech channel used in high frequency in the same hierarchy. A second characteristic is that the speech channels having high priority are repeatedly used by mobile stations near to the base station in question more frequently. A third characteristic is that the speech channels having low priority are used by mobile stations apart from the base station in question at a longer repeat interval.
A conventional dynamic channel assignment method applied to the hierarchical cellular type mobile communication system has a good characteristic in a case where the microcells are uniformly spread all over the service areas and/or in another case where speech is uniformly carried out. However, the conventional dynamic channel assignment method is disadvantageous in that speech in the microcell is pressured due to interference in the macrocell on initial introduction of the system where the microcells are not uniformly spread in all over the service areas or when speech is not uniformly carried out.